It boost sales, but not the way Sony wanted:

http://www.computerandvideogames.com/article.php?id=211019

This was supposed to be the game that would be ps3s Halo, now after a month on the market it is clearly not, it is obvious this game did not have a significant impact on ps3 sales.

Just to add I had earlier said this about the game:

http://www.vgchartz.com/forum/thread.php?id=62915&page=2

My final impressions on the game is that Killzone 2 is a good game period, but is not the best shooter on the market, that title belongs to Gears of War.

Yakuza 3 moved more hardware than Killzone 2 did, but Killzone 2 still moved a fair bit- perhaps 30,000 consoles.

No it wasn't and realistically it was never going to be one. The only hardware movers there is, are the serious game franchises eg.Mario, Metal Gear, Zelda , Final Fantasy and a select few others. Killzone 2 is a decent game that only the most radical fanboys, honestly thought could move meaningful amounts of hardware.

not especially. TO me it seems alot like Gears 2. It moved a bit of hardware but alot of people interested in it already owned the system

 thats correct but i think in my faint memory i remember that gears 2 did give 360 a small boost in america.

yes of course it did it helped outsell the 360 for a week but i sadly excepted more

I would think that a game good enough to get people to buy the system, would be good enough for people who already own the system to get it in droves. That does not seem to have happened.